The present invention relates to a process for removing antimony from hydrofluoric acid solutions which contain Ta/Nb and have an acidity &gt;18N.
When preparing Ta and Nb products by digestion with hydrofluoric acid and subsequent purification with MIBK, antimony (Sb) is an undesirable constituent because it cannot be removed from the process by this method. It is found mainly in the Ta product, but it is also found in unacceptable concentrations in Nb.sub.2 O.sub.5. The actual extent of contamination depends on the concentration in the raw material.
Ta.sub.2 O.sub.5 and Nb.sub.2 O.sub.5 are being used to an increasing extent in high-tech fields such as optoelectronics, with the most stringent requirements placed on purity. That means that contamination with Sb has to be reduced to less than 5 ppm. However, the largest Ta.sub.2 O.sub.5 raw material deposits in the World, in Australia, contain a large amount of antimony. Low-antimony raw materials are steadily running out or are no longer obtainable. It is therefore becoming increasingly important to provide a method for removing the Sb. Raw materials with up to 500 ppm of Sb.sub.2 O.sub.5 are generally used in the fluoride process, wherein the antimony then appears at this order of magnitude in the resulting Ta.sub.2 O.sub.5. It is reduced to a tenth of this in Nb.sub.2 O.sub.5.
It is known that antimony in raw materials can be removed by pyrometallurgical processes, but these methods are not economically viable. Furthermore, it is known that Sb can be reduced in dilute mineral acids such as hydrochloric acid using base metals. This process is not known in the case of hydrofluoric acid, in particular at the concentrations which are conventionally produced during the digestion of Ta/Nb-containing raw materials. In this process, hydrofluoric acid concentrations of &gt;18N are used. Under these conditions the antimony is retained in solution or redissolved. In trial examples, given below, relating to the solubility of Sb in HF-containing solutions, surprisingly, a solubility for metallic antimony was found which conflicts with indications of the known prior art.
Furthermore, the working-up of antimony-containing Ta/Nb ores is rendered difficult because antimony cannot be brought into solution by using mineral acids, with the exception of hydrofluoric acid. Thus separation via a chemical route, by means of fractionated dissolution using, different mineral acids, is not possible. The object of the invention is, therefore, to provide a process which enables antimony to be removed from strong hydrofluoric acid solutions before extraction with MIBK.